It's not going to be a fixed number of hours. It will depend on your voice and if you are taking it nice and easy, pushing to the max, or just where in between you are practicing. In other words, it depends on exactly How you are practicing.
It is important to practice as much as you can manage to, but with a sensitive care to stop any time you think you may be overdoing it. Chances are if you think you may be straining or overdoing stretches on your voice, then you already are.
Try to learn to listen to what your body and your voice are telling you, and don't push too far. If you are moderate in your practice, then there is practically no limit to how much singing you can do. Stay hydrated, and don't go to extremes. Most of us are in such a hurry to achieve our goals that we exceed our current capabilities and get ourselves into trouble.
You're in charge of how hard you push. Take it slow and easy, steady as she goes. The more you do in moderation, the longer you can stay with it.